1. Field of the Invention
The invention relates to thin-film solar cells, as well as to a process for the manufacture of thin-film solar cells.
2. Discussion of the Background
It is known that photovoltaic solar cells built on a support comprise a front or window electrode, an absorber layer and a rear electrode. In general, and in what follows, the electrode through whose plane the light to be converted to voltage or electrical power respectively penetrates into the absorber layer is referred to as the window electrode. The window electrode must therefore be as transparent as possible or must have high light transmission, in order that it does not needlessly reduce the efficiency of the solar cell. On the other hand, the rear electrode provided on the other face of the absorber layer can be relatively thick and opaque. It must be characterized substantially by the lowest possible surface electrical resistance and good adherence to the absorber layer and, as the case may be, to the underlying layer. In most cases, the rear electrodes are manufactured from molybdenum metal, which satisfies the foregoing conditions.
In the most widely used type of thin-film solar cells, the rear electrode is disposed between a support, the underlying layer, and the absorber layer; the transparent window electrode is disposed on the cell face situated opposite the underlying layer. Consequently, the underlying layer likewise does not necessarily have to be transparent. It can be made of glass, ceramic, polymer films or even of metallic sheets.
In solar cells with an overlying layer, the window electrode is disposed between the support, which in this case must also be highly transparent and, as the case may be, poorly reflecting antireflective, and the absorber layer, such that the light reaches the absorber layer through the support and the window electrode. In this case, the rear electrode situated opposite the support does not have to be transparent.
The absorber layer is most often made of a layer of chalcopyrite with additions of copper, indium and selenium (known as CIS absorber layers), sometimes also with sulfur instead of selenium. Occasionally the absorber layer is also doped with gallium (CIGS absorber layers). The absorber layer generally exhibits p-type conduction. To manufacture a pn junction, a buffer layer of material having n-type conduction is applied in a thickness of less than 100 nm on the absorber layer having p-type conduction. It is known from U.S. Pat. No. 4,611,091 that cadmium sulfide (CdS) can be used as material for the buffer layer, with a conductive window of ZnO placed thereabove.
If zinc oxide (ZnO) or another transparent oxide is used as material for the window electrode, this material, which is dielectric in itself, must be deposited as a doped semiconductor. The conductivity is achieved by doping, with aluminum or boron among other substances. On the industrial scale, these window electrodes are most often deposited by sputtering (cathodic sputtering under a magnetic field) on the surface of the absorber layer. However, layers with a thickness of 400 nm and more then are needed in order to limit the surface resistance to a usable level. Consequently, however, the light transmission is reduced compared with thinner layers. Another drawback of this process is that the sputtering parameters, in particular the oxygen partial pressure in the reactive atmosphere of the sputtering chamber, can be variably adjusted only in a very narrow range in order to obtain optimal results. Finally, the deposition of relatively thick ZnO layers is also time-consuming and costly because of a relatively low rate of coating with zinc metal in a reactive atmosphere. As an alternative technique there can be used ceramic targets, which are already composed of the desired conductive zinc oxide. Nevertheless, there is no advantage as regards deposition rate.
It is certainly possible to produce a ZnO window electrode with results that are also still usable from the optical viewpoint, by chemical vapor deposition (CVD), but even thicker layers up to 1500 nm must be tolerated in order to obtain satisfactory conductivity by this process, because the material density of the layers produced in this way is inferior to that of layers deposited by sputtering.
It has also been observed that a relatively thin layer (such as 100 nm) of dielectric ZnO between the absorber layer and the window layer of ZnO made conductive by doping increases the efficiency of the solar cell, and that it also has a positive influence on the stability of the process.
An advantage of this configuration, however, is that the known solar cells with CIS absorber layers have an open-circuit potential which depends on a difference of charge between the absorber layer with p-type conduction and the ZnO electrode made conductive by doping (with n-type conduction).
The object of the invention is to provide a process for the economic manufacture of solar cells with an improved window electrode and to propose thin-film solar cells equipped with such window electrodes.
Thus the window electrode can be manufactured with a thin metal-base layer, which is treated to be antireflective by at least one highly refractive oxide or nitride layer at least on the side on which light is incident.
With the use of a metallic layer or of a metal-base layer respectively, the conductivity of the window electrode is generally increased. With the antireflective treatment at least on the side of the metallic layer on which light enters (or in other words on its surface opposite the absorber layer), it is ensured that the usable light also passes effectively through the electrode and is not reflected for the most part or at all at the surface of the metallic layer.
In principle, it is of little importance whether or not the antireflective layer itself is electrically conductive. It may be deposited in a single layer or in a succession of layers, with the only limitations that, on the one hand, it must be sufficiently transparent and, on the other hand, it must adhere well to the metallic layer and be chemically compatible therewith.